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  <script>
    var age = 19
    // ++age   // age = age  +1

    // age++   // age = age + 1
    // age++   // age = age + 1

    // 后置： 先用用这个原先的值， 再将值自增 + 1
    // console.log(age++ + age++)  // 19 + 20  = 39

    // // console.log(age)  // ?21


    // // age = 21
    // console.log( age++ + age + ++age + age )  // 21 + 22 + 23 + 23 = 89

    // age--   // age = age -1
    // age--   // age = age -1

    // console.log(age + age--)  // 17 + 17 = 34
    // console.log(age)  // 16

  </script>
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